Present day carriage-type leg press machines all work on the same primary principle of fixing the positions of either the operator's ankle joints or the operator's hip joints while applying a resistive force, through the use of an operator-engaging guided-carriage assembly, to the operator's opposite nonconstrained pair of hip or ankle joints which tends to make these two pairs of joints move toward each other along the lines between them. The operator's leg extending muscles (quadriceps femoris) and hip extending muscles (gluteus maximus) are developed as they oppose this force by tending to move these two pairs of joints apart during the exercise movement.
It is a well known fact and easily verified that due to joint mechanics, angles of pull of muscles, physiological make-up of muscles, etc. that in a leg press movement (combined hip and knee joint extension) more force can be applied as the legs become more extended to the point of lockout. Consequently, a leg press machine which varies the resistive force applied to correspond with the positionally related strength capabilities of the operator's leg pressing muscles will be more effective at developing those muscles.
The standard method of varying the resistive force applied on present day carriage-type leg press machines is through the use of cams used in conjunction with chains or cables, all of which have inherent problems. The problem with cams is that they are relatively hard to manufacture. The problem with cables is that because of their relatively small cross sectional area they carry very high tensile stresses (a 5/8" cable carrying 200 lbs, for example, has a tensile stress in it of approximately 16,300 psi). These already high stresses are multiplied and become cyclic (introducing fatigue wear) when a cable moves along bending over a small diameter pulley. These high cyclic stresses applied to relatively small cross sectional areas make cables stretch (eventually decreasing the machine's intended range of motion) and eventually fray and wear out (leading to replacement or catastropic failure). Chains, while not suffering the fatigue wear that cables do, are subject to stretching at their many joints (thus decreasing the machine's intended range of motion). They, also like cables, are subject to relatively high tensile stresses and in addition are noisy and introduce spurious drag to the machine.
In view of the advantage of applying a variably resistive force to an operator's leg pressing muscles in a leg press exercise movement, and the disadvantages of obtaining such a force through the use of cams, chains, or cables, it is the objective of the disclosed invention to introduce a carriage-type leg press machine which applies a pre-determined variably resistive force to an operator's leg pressing muscles through the use of a force-varying mechanism which uses only rigid members and pinned joints, thereby eliminating the problems associated with force-varying mechanisms using cams, chains, or cables.